The increased mobility of society and the need for maintaining swift and efficient communications has resulted in the development of paging systems. Paging systems are usually divided into two distinct groups; on-site and area-wide. The area-wide paging system is used for contacting personnel over large geographic areas. The on-site paging systems are used for locating personnel within a particular building or facility. A typical use of on-site paging systems include the location of personnel in hospitals and high security institutions such as military, installations and prisons.
Presently available on-site paging systems generally include a network of loudspeakers, lamp displays or radio units. These paging systems are typically of the broadcast variety and usually do not know the location of the paged party.
Another technique for locating and contacting personnel consists of scanning a transmitting unit, on the person to be located, to monitor the location of the person at all times. These personnel locating systems usually include an ultrasonic transmitting unit, carried by the person to be located, which are either scanned or manually activated to transmit a unique ultrasonic signal to a network of receivers distributed throughout a building. With these personnel locating systems since the location of personnel is known at all times any need for paging can be eliminated or highly selective in scope.
Typical applications of ultrasonics in personnel locating systems are described in U.S. Pat. Nos. 3,439,320; 3,696,384 and 3,739,329.
These prior art ultrasonic personnel locating systems (PLS) include a portable battery-powered transceiver that is periodically scanned and which responds to signals from remote receiving locations throughout the facility. Since the transceiver operates only when scanned, power consumption is kept low. Utilizing the scan/respond transmission format results in the transceiver sometimes being in a "dead zone" during the scan cycle and hence a "temporary lost personnel" status would result. Additionally, since the distance from the transceiver unit to the remote receiver location is variable the transmitter design must accommodate worst case transmission ranges at the expense of additional power requirements.
Another problem exists with these prior art ultrasonic personnel locating systems due to their susceptibility to ultrasonic noise. In most applications there is heavy ultrasonic background noise which interferes with the operation of the ultrasonic personnel locating systems. Additionally, ultrasonic signals are easily reflected by walls, cabinets, furniture, etc., which provides multipath signal interference. Since ultrasonic signals travel only at the speed of sound, the longer propagation delay of the reflected signal produces a more troublesome multipath interference signal. Finally, the limited bandwidth of ultrasonic signals limits the number of unique codes that can be assigned to the portable units.
Prior art infrared techniques include the use of an infrared transmitter to communicate analog music information from a music receiver or amplifier to a portable infrared headset receiver. The infrared transmitter in such a design is not portable and utilizes energy inefficient analogue modulation techniques. Since the portable receiver merely detects the signal and drives an efficient close coupled headset to provide music to the listener much less power is required than for a portable transmitter design.
Other prior art infrared techniques include the use of an infrared battery powered transmitter for remote control of the channel selection volume and on/off controls of a television receiver. Since these controls are used only when manually activated by the user, the transmission can be inefficient and yet still utilize low battery power.
The application of these known infrared techniques to a portable battery-powered transmitter for use in a personnel locating system design would require a manual operation by the user each time he entered a room.
It is therefore a problem to design an infrared personnel locator system having a low power dissipation battery-powered portable transmitter which transmits a periodic unique infrared identification code identifying the person carrying the transmitter unit to a plurality of remote infrared receiver units.